Screw press useful in the manufacture of alkali cellulose and cellulose ethers

ABSTRACT

This application discloses a new modification of a two-screw press of the known type in which the screws are of the Mapre type, i.e. have a rectangular thread profile. The modification concerns the depth of the thread. Its use is advantageous for promoting the reaction of fibrous cellulose with alkali, or with alkali and with a liquid or solid etherifying agent, or for promoting the reaction of fibrous alkali cellulose with a liquid or solid etherifying agent.

United States Patent Division of Ser. No. 786,843,

Dec. 20, 1968, now abandoned, which is a continuation of applicationSer. No. 547,659, May 4, 1966,

now abandoned.

[54] SCREW PRESS USEFUL IN THE MANUFACTURE OF ALKALI CELLULOSE ANDCELLUlLOSlE ETHERS 3 Claims, 1 Drawing Fig.

52 vs. (II 23/285, 18/12 SP, 25/14, 259/6, 259/104, 260/231, 260/232[51] lint. Cl BOlj 1/00, 82% 1/10, C08b 11/20 [50] Field of Search23/285, 252, 290; 259/6, 104: 18/12 SP, 30 SM; 25/14;

[56] References Cited UNITED STATES PATENTS 2,115,006 4/1938 Burghauser259/6 2,434,707 1/1948 Marshall 259/6 X Primary Examiner-JosephScovronek Attorney-James E. Bryan ABSTRACT: This application discloses anew modification of a two-screw press of the known type in which thescrews are of the Mapre type, ie have a rectangular thread profile. Themodification concerns the depth of the thread. Its use is advantageousfor promoting the reaction of fibrous cellulose with alkali, or withalkali and with a liquid or solid etherifying agent, or for promotingthe reaction of fibrous alkali cellulose with a liquid or solidetherifying agent.

PATENTEDucT 26 m1 3,615,254

2 i M 1 x W INVEN FRANZ EICHENS By HANS KLETSCHKE ATT EY SCREW PRESSUSEFUL IN THE MANUFA'CTURIE Ol ALKAlLll CELLULOSE AND CIEILIL UILOSIEIETHERS CROSS-REFERENCE TO RELATED APPLICATIONS The present applicationis a division of copending US. Pat. application Ser. No. 786,843, nowabandoned filed Dec. 20, 1968, which is a continuation of US. Pat.application Ser. No. 547,659, filed May 4, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention is concerned withimprovements in the manufacture of alkali cellulose and celluloseethers.

It is known to produce cellulose ethers by reacting alkali cellulosewith suitable solid or liquid etherifying agents. The alkali celluloseiequired for this purpose has been obtained in a separate operation byreacting cellulose or materials containing cellulose with alkali,especially sodium hydroxide. Thus, the cellulose ethers are in this caseproduced in a twostage process.

It is moreover known to produce cellulose ethers in a singlestageprocess by reacting cellulose or materials containing cellulose withsolid or liquid etherifying agents in the presence of alkali.

Both processes are performed on an industrial scale. The single-stageprocess has the advantage of requiring less equipment, while thetwo-stage process has the advantage of greater variability, since themolecular weight and the distribution of the molecular weight of thealkali cellulose can be controlled by suitable reaction conditionsduring the preparation thereof. The common feature of the two processesis that a fibrous or fibrous and friable solid substance must bereacted. This circumstance, above all, makes it difficult to avoid usingmore than the equivalent quantity of any reactant other than thecellulosic reactant.

The aim of carrying out the process with the smallest quantity of otherreactant that is theoretically necessary cannot be completely achievedeither in the reaction of alkali cellulose with etherifying agents togive cellulose ether or in the reaction of cellulose with alkali andetherifying agents to give cellulose ether. Generally, the process iscarried out in the presence of comparatively small amounts of liquid, sothat the reaction mixture forms a loose, friable, fibrous mass. In sucha mixture the reaction proceeds comparatively slowly and considerableamounts of the etherifying agent are eliminated from the reaction byhydrolysis. In addition, the reaction products do not meet with therequirements of uniformity.

Accordingly, in order to increase the reaction velocity and to obtain auniform product, the reaction has been carried out in kneading devices,but products sufficiently uniformly etherified have not been obtained inthis way; in addition, continuous operation is not possible.

A substantial acceleration of the etherification process and thepossibility of continuous operation were first achieved by the use ofscrew presses. At the present time, both singlescrew and multiscrewpresses are used for the continuous etherification of alkali celluloseor of cellulose with alkali and etherifying agents. When single-screwpresses are employed, however, it must be borne in mind that theconveying capacity depends greatly on the liquid content of the mediumbeing conveyed. The quality of the cellulose ether obtained accordinglybecomes dependent on the nature of the alkali cellulose, so thatcontinuous supervision of the process is required. The use of mulitscrewpresses, however, also has not so far given fully satisfactory results.Their conveying capacity is substantially independent of the moisturecontent of the material to be conveyed and homogenized, but the intakecapacity of the screws in the intake or feed portion is subject tovariations in dependence upon the quality of the starting material.Moreover, the quality of the etherification product that can be obtainedin laboratory operation when the reaction mixture is passed through ascrew press several times, cannot be attained in industrial practicewith the multiscrew presses hitherto employed.

The above-mentioned object of achieving the reaction of cellulose or itsderivatives as far as possible with only the equivalent quantity of anyother reactant cannot, however, be attained in the production of alkalicellulose from cellulose with alkali. Mixture of cellulose with only asmall excess of concentrated aqueous sodium hydroxide also are of afibrous and friable nature, but it has not been possible hitherto tohomogenize them by means of single-screw or multiscrew presses.

The intake capacity of single-screw presses for this mixture is poor andthey also convey it unsatisfactorily. They cease to convey it altogetherif a pressure gradient builds up within the press. The mixture ofcellulose and alkali then becomes jammed in the threads of the screw andis moved only in a circle. On the other hand, in the case of multiscrewmachines a more or less adequate conveying capacity is initiallyachieved if the material to be conveyed is not too dry. Nevertheless,homogenization of the medium has not been successfully achieved at all.This is a requirement, however, since otherwise the conversion intoalkali cellulose proceeds too slowly in the tightly compressed material.

Alkali cellulose is therefore generally produced by immersion of sheetsof cellulose or endless webs of cellulose in an excess of cold or hotsodium hydroxide solution and then squeezing out. Screw centrifuges areused for this purpose in order to obtain at least partial homogenizationduring the squeezing-out process.

Therefore, the problem has hitherto existed to produce a homogenizingand conveying means with which fibrous and friable reaction mixtures ofcellulose and alkali cellulose and etherifying agents, or of celluloseand alkali and etherifying agents, could be homogenized and conveyed sothat the corresponding reactions could be accelerated.

It was to be expected that, in spite: of the multiplicity of negativeresults, this object most likely could be achieved by the use of atwo-screw or multiscrew press, since in these presses high shearing andcompressive forces occur which should be capable of effectinghomogenization of the material if the flow properties of the materialand the characteristics of the screw could be suited to one another.However, the action of the known screw presses is in general totallyinadequate for the homogenization and transport of fibrous and friablereaction mixtures such as occur especially in the manufacture of alkalicellulose and cellulose ethers. Merely, a two-screw press of the knowntype in which the screws are of the Mapre type, i.e. have a rectangularthread profile and are counterrotated with respect to each other, provedto be: applicable.

SUMMARY OF THE INVENTION The invention is concerned with a newmodification of the known two-screw press of the Mapre type in which thescrews interengage with a housing, part of which is occupied by anintakev The modification concerns the depth of the thread. Moreprecisely, the modification consists in an inequality of the thread ofthe interengaging screws such that each of them has a greater depth ofthread in the part of the housing not occupied by the intake than in thepart occupied thereby. Hence, generally spoken, the object of theinvention is a press including a housing, said housing having an intakeopening formed therein, two screws of substantially uniform outerdiameter, substantially continuous rectangular thread profile andsubstantially constant thread flank spacing throughout their length, andbeing supported within said housing, means for rotating one screw in aparticular direction, means for coun terrotating the other of saidscrews with respect to the one, said screws being mirror images of oneanother and being engaged with one another, said screws having a certaindepth of thread adjacent said intake opening and having a greater depthof thread at a point spaced from said intake opening to define aclearance between the screws. The two-screw press of the invention ismore apt to promote reactions of fibrous cellulose materials with liquidor solid reactants than known screw presses of the Mapre type.

A two-screw press of the invention is illustrated in the accompanyingdrawing, the single figure of which is a plan view of the press with theintake and a part of the shell of the housing removed.

BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates a longitudinalcross section through a press according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thetwo screws 1 and 2 of rectangular profile 3 which are images of oneanother are mounted in a housing 4 which in cross section has the formof a figure eight and which has an intake opening 5. Within the housing4, the screws have a clearance 6 with respect to one another, while inthe intake portion the clearance between the screws is made very muchsmaller. The screw shafts rotate in opposite directions indicated by thearrows 7 and 8. The outlet of the double-screw press in not shown in thedrawing. The press itself is of customary double-screw press design. Theoutlet opening may be closed by a sieve plate by which the finalpressure in the double-screw press can be regulated.

Friable, moist cellulose material is taken in a downward direction intothe housing on both sides of the intake opening by the rectangularprofile of the counterrotating screws. The smaller the clearance betweenthe screws, the more material is packed between the inner and outerperipheries of the two screws, which peripheries converge at the base ofthe housing, so that the empty screw space at the top again becomesavailable for conveying the material. Of all the profiles tested, therectangular profile showed the greatest intake effect. Within thehousing, owing to the somewhat large clearance between the screws, morevigorous longitudinal mixing and conveyance of the material occurs. Byemploying the Mapre screw press, as illustrated, it is possible tohomogenize in an excellent manner both friable, moist mixtures ofcellulose and alkali hydroxide and also reaction mixtures for producingcellulose ethers. Thus, cellulose ethers of very good quality can beproduced from alkali cellulose by admixing appropriate liquids, forexample hydroxypropyl cellulose by admixing propylene oxide, orcarboxymethyl cellulose by admixing monochloracetic acid, or by admixingappropriate solids, such as monochloracetate in the preparation ofcarboxymethyl cel lulose.

It is advantageous to effect a coarse admixture of the reactants beforethey enter the Mapre double-screw press, in order to facilitate theaction of the press. However, if the individual reaction constituentsare supplied in a sufiiciently uniform manner, it is possible to employthe double-screw press for the entire mixing process. As already hasbeen stated, the reaction mixture should be fibrous and friable when itenters the double-screw press. This means that the reaction mixture isnot wet, but at most it is moist to the touch. When cellulose is mixedwith sodium hydroxide, this desired state is reached when l-2 parts byweight of sodium hydroxide solution are used to about one part by weightof cellulose. In the production of cellulose ethers, irrespective ofwhether they are prepared directly from cellulose or from alkalicellulose, about one to 1.5 parts by weight of liquid can be used withadvantage to one part by weight of cellulose or alkali cellulose.

Not only is it possible to carry out the homogenization of the reactantsby means of Mapre screw presses, but completely orpartially theconversion into the desired final product also can take place directlyin the screw press. In the latter case, it is particularly advantageousto provide the screw press with suitable heating or cooling devices. Inthis way, regulation both of the reaction velocity and of the molecularsize of the final product can be effected. Both the housing and thescrews are advantageously double walled and are heated or cooled. Thescrews and the housing are heated, when, for example, it is desired toobtain some ripening at the same time during the preparation of alkalicellulose. In this way, by suitable relative adjustment of heating anddwell time, the desired degree of ripening can be obtained. When, on theother hand, retention of a particular molecular size is required, theheat of reaction and the frictional heat are removed by cooling.

The external dimensions of the screw press must be adapted to thedesired degree of homogenization and to the throughput that is required.It is in general the case that a large capacity and a long dwell timeare required when the conversion of the reactants is to be fairlyexhaustive. In this case, the apparatus may be equipped with a low powerdrive. A higher power drive is required when the screw press has a smallinternal capacity and the material to be put through is to have only abrief dwell time.

It is generally advisable to make the depth of the threads of the screwsrelatively great. The compression rate obtained by means of the screwshould be 3:1 at the maximum. The final pressure at the outlet of thescrew press should not be more than 300 atmospheres gauge. At present,maximum throughputs of 75 kg. per hour with a screw diameter of 92 mm.and a speed of 22r.p.m. can be obtained with Mapre screw presses.

The advantageous operation of Mapre presses in comparison with otherscrew presses, in particular double-screw presses, is illustrated in thefollowing examples.

EXAMPLE l 15 kg. of coarsely shredded alkali cellulose with a cellulosecontent of 45 percent by weight and a sodium hydroxide content of 16percent by weight were intensively mixed for 5 minutes with 3.5 kg. ofsodium chloracetate in a mixer. The resulting mixture was then pressedthrough a commercial double-screw press of the Mapre type with a screwdiameter of 92.6 mm. and a screw length of 742 mm. The speed of thescrews was 8 r.p.m. and the throughput was 23 kg./hour.

The reaction time in the double-screw press was 15 minutes at atemperature of 60 C. The mixture was then subjected to an afterreactionproduct was efiected in the conventional manner. The etherificationproduct obtained in this way, in 2 percent aqueous solution at 20 C.,had a viscosity of 260 cp. (centipoises) and a solution residue of l 1percent by weight.

When, on the other hand, a reaction mixture prepared in the same way waskneaded in an intensive kneader for 60 minutes at 60 C. in the hithertocustomary manner, an etherification product was obtained which had aviscosity of only 113 cp. and a water-insoluble residue of 22 percent byweight.

This shows that considerably improved products are obtained with theMapre double-screw press.

EXAMPLE 2 10 kg. of cellulose powder were mixed with 20 kg. of 45percent sodium hydroxide for about 15 minutes in a stirrer. Theresulting mixture was passed once through a Mapre doublescrew press ofthe special form described above and the dimensions of which were asfollows: Screw diameter 92 mm., screw length 850 mm., width of thread 22mm., depth of thread in the intake portion 24 mm., depth of thread inthe housing 24.5 mm.; clearance between the screws in the intakeportion: the screw shafts bore tightly on one another; clearance betweenthe screws in the housing 1 mm., pitch of the screws 40 mm., speed ofthe screws 8 r.p.m. throughput 30 kg./hour.

The resulting homogenized mixture was left to stand for about another0.5 hour, so that the reaction could be completed. For the purpose oftesting the homogeneity of the product, etherification thereof withmethyl chloride was then carried out. A measure of the uniformity of theconversion is the viscosity and the water-insoluble residue of theether.

With a 2 percent solution of the ether, a viscosity of 1500 cp. wasmeasured at 20 C., 1.5 percent by weight of the ether was insoluble inwater.

For comparison purposes, a reaction mixture prepared in the same way waspassed through a double-screw press in IAIM'I IIIIK which the screwsalso rotated in opposite directions but had a chamfered profile.

To obtain the same throughput as in the above-mentioned case, the threadvolume of this double-screw press had to be selected about 20 percentlarger. The speed had to be increased to 15 r.p.m. The homogenizedmixture was again reacted with methyl chloride in this case and theviscosity of the etherification product and the residue insoluble inwater were determined. A viscosity of 1200 cp. and an insoluble residueof percent by weight were found.

The same test was repeated once more, this time using screws rotating inthe same direction and also with a roundedoff self-disengaging profile.in this case, a screw thread volume about 30 percent larger than in thecase of the double-screw press according to the invention was required.in addition, the speed had to be increased to 14 r.p.m., which can beattributed to the direction of rotation being the same. The methyl etherobtained by etherification of the alkali cellulose produced in this way,in 2 percent aqueous solution at 20 C., showed a viscosity of 1000 cp.and a residue of 7 percent by weight.

This shows clearly that a substantially more uniform product with anincreased throughput can be obtained with the special Mapre double-screwpress.

It will be obvious to those skilled in the art that modifications may bemade within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

What is claimed is:

11. A press including a housing, said housing having an intake openingformed therein, two screws of substantially uniform outer diameter,substantially continuous rectangular thread profile and substantiallyconstant thread flank spacing throughout their length, and beingsupported within said housing, means for rotating one screw in aparticular direction, means for counterrotating the other of said screwswith respect to the one, said screws being mirror images of one anotherand being interengaged with one another, said screws having a certaindepth of thread adjacent said intake opening and having an about 2percent greater depth of thread at a point spaced from said intakeopening to define a clearance between the screws.

2. Apparatus as defined in claim 11 wherein said housing has across-sectional configuration substantially in the form of a figure 8.

3. Apparatus as defined in claim 1 wherein the compression rate obtainedby means of the screws has a maximum value of about 3:1.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION t n 3 615 Z54 D teOctober 26 1971 Inventor(s) Franz Eichenseer et al It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 16, "iequired" should read required Column 2, line 5,"Mixture" should read Mixtures Column 4 line 37 after "product" at 60 OC. over a period of 48 minutes. The working up of the reaction should beinserted.

Signed and sealed this 28th day of March 1972.

(SEAL) Attest:

EDNARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents -1050 10-69 RM PO USCOMM'DC B0376-P69 s LI 5 GOVERNMENYPnnmuc OFFICE 1959 o-3ss a:u

2. Apparatus as defined in claim 1 wherein said housing has across-sectional configuration substantially in the form of a figure 8.3. Apparatus as defined in claim 1 wherein the compression rate obtainedby means of the screws has a maximum value of about 3:1.